There is a known gas concentration detection apparatus having an oxygen pump cell for discharging excess oxygen and a sensor cell for detecting the concentration of a specific component in a gas from which the excess oxygen is discharged (refer, for instance, to Patent Document 1). The gas concentration detection apparatus disclosed in Patent Document 1 corrects the value of a current flowing in the sensor cell in accordance with the value of a current flowing in the oxygen pump cell. This makes it possible to achieve concentration detection with high accuracy even when the concentration of oxygen in a detection target gas varies.
The oxygen pump cell and sensor cell each include an element made of a solid electrolyte. A gas concentration detection apparatus having an element made of such a solid electrolyte needs to raise the temperature of the element to a predefined activity temperature. There is another known apparatus that detects element resistance (element impedance) and exercises heater energization control in accordance with the deviation between the detected element resistance and a target value (refer, for instance, to Patent Document 2). The apparatus disclosed in Patent Document 2 makes it possible to maintain a desired element temperature by correcting the target value for the element resistance in accordance with heater power.
There is still another apparatus that forms an activity judgment about a gas concentration sensor in accordance with element impedance (refer, for instance, to Patent Document 3).
A gas concentration detection apparatus disclosed in Patent Document 6 corrects the value of a current flowing in a sensor cell in accordance with the value of a current flowing in a pump cell when the pump cell discharges excess oxygen from a detection target gas to change the concentration of oxygen in the detection target gas. This makes it possible to avoid an unexpected change in NOx concentration even when the concentration of oxygen in the detection target gas changes.
Patent Document 1:
JP-A-2002-116180
Patent Document 2:
JP-A-2003-50227
Patent Document 3:
JP-A-2004-177179
Patent Document 4:
JP-A-2005-91228
Patent Document 5:
JP-A-2001-141696
Patent Document 6:
JP-A-2000-137018
Patent Document 7:
JP-A-2001-74692
Patent Document 8:
JP-A-2000-171436